As shown in FIG. 3, Japanese patent Laid-Open No. 234681/1993 or the like discloses an organic EL element in which a transparent electrode 2, an organic layer 3, a back electrode 4 are successively formed on a translucent insulating substrate 1, and the organic layer 3 is constituted by, for example, a hole injecting layer, a hole transporting layer, a light emitting layer, and an electron transporting layer.
In the organic EL element as stated above, a direct current power supply of several tens of volts is connected to the transparent electrode 2 and the back electrode 4 to supply an electric current between the electrodes 2 and 4, so that the organic layer 3 emits light, and the light is irradiated toward the outside through the substrate 1. Its light emitting portion is generally determined by the pattern of the transparent electrode 2, except for a dot matrix type.
That is, as shown in FIG. 4, a display portion D and a wiring pattern L for supplying an electric current to this are patterned and are formed on the substrate 1 of glass or the like by using a translucent conductive material such as ITO and by a method such as vapor deposition. The display portion D is formed to be a size larger than the segment portion (region indicated by a dotted line in FIG. 4) S corresponding to the emission in a specific shape, and as shown in FIG. 3, an insulating layer 5 formed by a suitable measure, such as spin coating or roll coating, using an insulating material such as polyimide, acrylic, or phenol, is positioned between the transparent electrode 2 and the organic layer 3. The part of the display portion D other than the segment portion S (the part covered with the insulating layer 5) is a part of the wiring portion L, and forms the insulating layer 5 except for the emission in a specific shape. That is, by removing the insulating layer 5 on the segment portion S, the shape of the segment portion S has been made the desired emission in a specific shape.
In the organic EL element as stated above, for example, as shown in FIG. 4, in a case where seven segment portions S1 to S7 are arranged in the shape of a the numeral 8 to express the figures 0 to 9, display portions D1 to D7 including the segment portions S1 to S7 and wiring portions L1 to L7 respectively connected to them are formed, and current supply sources (not shown) are separately connected to the wiring portions L1 to L7.
Incidentally, in a case where a pattern is adopted such that the segment portions S1 to S6 are arranged clockwise, the segment portion S7 is arranged at the center, and all the wiring portions L1 to L7 connected to these are led out downward, the wiring portions L1, L2, and L6 (former group) of the segment portions S1, S2 and S6 positioned above have the extension distances longer than the wiring portions L3 to L5 and L7 (latter group) of the segment portions S3 to S5 and S7 positioned below, and the capacitance becomes larger.
By this, there is a case where the ratio of the capacitance of the segment portions S1 to S7 to the capacitance of the wiring portions L1 to L7 is much different between the former and the latter, and since it is known that an emission rise time depends on the capacitance greatly (for example, see column 0026 of Japanese Patent Laid-Open No. 29080/1993), and there are cases where differences in the emission rise time occur among the segment portions S1 to S7.
The difference of the emission rise time converts into difference of emission luminance in the segment portions S1 to S7, and the display quality is lowered. Especially in the case of such a structure that an electric current is supplied by pulse driving to control emission/non-emission of the segment portions S1 to S7, when the frequency of the pulse or the duty ratio is changed to adjust the emission luminance to a desired value, in the case where the frequency is high or the pulse width is short, there is a case where the segment portions S1, S2, and S6 connected to the former and having a long emission rise time do not emit light.
In the present invention, attention is paid to such problems, and its object is to provide an organic EL element which can realize an improvement in the difference of emission luminance due to the difference of an emission rise time in a plurality of light emitting portions.